Resin-coated steel sheet for drawn-and-ironed cans and drawn-and-ironed cans manufactured therefrom

ABSTRACT

A resin-coated steel sheet for drawn-and-ironed cans and a method of producing the resin-coated steel sheet. The resin-coated steel sheet has a steel sheet with a tin-coat on each side and a resin layer of a first crystalline thermoplastic polyester resin on one of the tin-coats and a second non-crystalline thermoplastic polyester resin on the first crystalline thermoplastic polyester resin. The first crystalline thermoplastic polyester resin has an alloying ratio of: 50&lt;/=alloying ratio &lt;100. The second non-crystalline thermoplastic polyester resin has an alloying ratio of 0&lt;alloying ratio &lt;50. The method includes thermal fusion of a mixture of 95-5 percent by weight of a crystalline polyester resin and 5-95 percent of a non-crystalline polyester resin to produce the first crystalline thermoplastic polyester resin and the second non-crystalline thermoplastic polyester resin. The first crystalline thermoplastic polyester resin and the second non-crystalline thermoplastic polyester resin are coated onto the tin-coated steel sheet using two-layer extrusion T-die and then cooled quickly.

This application is a continuation of now abandoned application, Ser.No. 07/875,593, filed Apr. 28, 1992, which is a continuation of nowabandoned application, Ser. No. 07/612,440, filed Nov. 14, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to a resin-coated steel sheet for drawnand ironed cans, and a drawn and ironed can manufactured therefrom.

2. Description of the Related Art:

Japanese Laid-open Patent Application No. Sho-60-168643 discloses aresin coated steel sheet for drawn and ironed cans (hereinafter referredto as "DI cans") which is excellent in workability in drawing andironing and which enables a high degree of ironing .(an ironing ratio of60% or higher) with a resin film being kept coated on the inside face ofthe can, and which gives, by drawing and ironing, DI cans satisfactoryin resin film adhesion, corrosion resistance, and external appearance.

The resin-coated steel sheet disclosed in the above Laid-open PatentApplication comprises (1) a coating layer of a thermoplastic resin suchas a polyethylene terephthalate resin, being capable of orienting andhaving a barrier property against corrosive components, the layer beingprovided on one side of the steel sheet, which side forms an inside faceof the DI cans; (2) an inorganic oxide film layer such as of chromiumhydrate oxide provided under the coating layer as an adheringundercoating; and (3) a metal coating layer of ductile metal such as tinprovided on the other side of the steel sheet which forms the outsideface of the DI cans.

In order to manufacture a DI can which has a resin coating satisfactoryin adhesion, corrosion resistance, and external appearance, as describedin the above Laid-open Patent Application, the coating resin of theresin-coated steel sheet for DI cans is required to be brought to asuitable drawing temperature in the drawing and ironing step, (thesuitable temperature being lower than the crystallization temperature ofthe resin, and within the range of the glass-transition temperature(T_(g)) ±30° C., e.g., 40° to 100° C. for a PET resin. Since theresin-coated steel sheets, which are produced by a knownextrusion-lamination method (T-die method) and subjected to the drawingand ironing, are usually stored at a normal temperature, a preheatingprocess for the resin-coated steel sheets is required, disadvantageouslyto be added to the conventional can-manufacturing process.

SUMMARY OF THE INVENTION

The present invention intends to provide a resin-coated steel sheetwhich has excellent workability in drawing and ironing even at a normaltemperature without preheating, and is capable of being ironed in a highdegree of ironing (an ironing ratio of 60% or more) while the resin iskept coated on the inside face, and which gives, by drawing and ironing,DI cans having resin coating satisfactory in adhesion, corrosionresistance, and external appearance.

The present invention also intends to provide a DI can having theaforementioned characteristics.

According to one aspect of the present invention, there is provided aresin-coated steel sheet for DI cans, comprising a steel sheet having atin coating on one side and a chromate-treated surface on the otherside, the chromate-treated surface being further coated with a resincomposition composed of a .thermoplastic polyester resin, the polyesterresin having been prepared by thermal fusion reaction of 95 to 5% byweight of a crystalline polyester and 5 to 95% by weight of anon-crystalline polyester and having an alloying ratio, represented bythe formulas below, of 5 to 50%. ##EQU1## where T_(m1) and T_(g1) arerespectively a melting point, and a glass-transition temperature, in °C., of the crystalline polyester resin employed as the startingmaterial; T_(m2) and T_(g2) are respectively a melting point and aglass-transition temperature, in ° C., of the completely reacted productof a thermal fusion reaction of the crystalline polyester resin with thenon-crystalline resin employed as the starting materials, or in otherwords a melting point and a glass-transition temperature of a randomcopolymer of the polyester resin having the same monomer composition;T_(m3) and T_(g3) are respectively a melting point and aglass-transition temperature, in ° C., of the thermoplastic polyesterresin.

According to another aspect of the present invention, there is provideda resin-coated steel sheet for DI cans, comprising a steel sheettin-coated on both sides, one side of the sheet being .further coatedwith a resin composition composed of a thermoplastic polyester resin,the polyester resin having been prepared by a thermal fusion reaction of95 to 5% by weight of a crystalline polyester with 5 to 95% by weight ofnon-crystalline polyester and having an alloying ratio, represented by,the formulas above, of 20 to 80%.

According to still another aspect of the present invention, there isprovided a resin-coated steel sheet for DI cans, comprising a steelsheet tin-coated on both sides, one side of the sheet being coated witha non-crystalline polyester resin layer, and the non-crystallinepolyester resin layer being further coated with a resin layer composedof a thermoplastic polyester resin, the polyester resin having beenprepared by a thermal fusion reaction of a crystalline polyester withnon-crystalline polyester and having an alloying ratio, represented bythe formulas above, of less than 50% (excluding 0%).

According to a further aspect of the present invention there is provideda resin-coated steel sheet for DI cans, comprising a steel sheettin-coated on both sides, one side of the sheet being coated with aresin layer composed of a thermoplastic polyester resin, the polyesterresin having been prepared by a thermal fusion reaction of a crystallinepolyester with a non-crystalline polyester and having an alloying ratio,represented by the formulas above, of not less than 50% and less than100%, and the resin layer being further coated with another resin layercomposed of a crystalline polyester resin.

According to a still further aspect of the present invention there isprovided a resin-coated steel sheet for DI cans, comprising a sheettin-coated on both sides, one side of the steel sheet-being coated witha resin layer composed of a first thermoplastic polyester resin, thefirst polyester resin having been prepared by a thermal fusion reactionof a crystalline polyester with a non-crystalline polyester and havingan alloying ratio, represented by the formulas above, of not less than50% and less than 100%, the resin layer being further coated withanother resin layer composed of a second thermoplastic polyester resin,the second polyester resin having been prepared by a thermal fusionreaction of a crystalline polyester with non-crystalline polyester andhaving an alloying ratio, represented by the formulas above, of lessthan 50% (excluding 0%).

According to a still further aspect of the present invention, there isprovided a drawn and ironed can, produced by drawing and ironing theaforementioned resin-coated steel sheet for DI cans to process theresin-coated side into the inside of the can.

The values of T_(m1), T_(m2), T_(m3), T_(g1), T_(g2), and Tg₃ arerespectively a melting point or a glass-transition temperature measuredby a differential thermal analyzer (Perkin Elmer, Model 7) at aprogrammed temperature elevation of 10° C./min. Regarding the definitionof "crystalline" and "non-crystalline", a polyester exhibiting a peak ofthe melting point is defined as being crystalline, and a polyesterexhibiting only the peak of the glass-transition temperature orexhibiting neither a peak of the glass-transition temperature nor a peakof the melting point is defined as being non-crystalline.

The above-mentioned crystalline polyester includes polyethyleneterephthalate (hereinafter referred to as PET), polybutyleneterephthalate (hereinafter referred to as PBT), polyethylene naphthalate(hereinafter referred to as PEN), and copolymers thereof, but is notlimited thereto. Thermoplastic polyesters having a melting point of 200°C. or higher are included in the crystalline polyester.

The above-mentioned non-crystalline polyester includes polyethyleneisophthalate (hereinafter referred to as PEI), polyethyleneterephthalate copolyester comprising 30 mole % ofcyclohexane-dimethanol, polyethylene terephthalate comprising 20 mole %or more of isophthalic acid, polyarylate, polyester-polycarbonate, andthe like, but is not limited thereto. Non-crystalline thermoplasticpolyesters having a glass-transition temperature of 20° C. or higher,preferably 40° C. or higher are included thereto.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail.

The first and the second aspects of the present invention are explainedbelow.

Cans were manufactured by coating a variety of thermoplastic resins on achromate-treated surface of a known surface-treated steel sheet having atin coated surface on one side for the outside wall of the can, and thechromated-treated surface on the other side for the inside wall of thecan, or on one side of a known surface-treated steel sheet having tincoating on both sides prepared according to a known method, andsubsequently drawing and ironing the steel sheets thus resin-coated,with the resin-coated surface forming the inside of the can.

Firstly, a known PET was selected as the thermoplastic resin. Further, aknown resin in an amorphous state was employed. As a result, theresin-coated steel sheet prepared by a known method could be drawn andironed only to a low degree, but could not be worked to an ironing ratioof 60% or higher, the object of the present invention. The reason waspresumed to be that the usual non-crystalline PET singly usedcrystallizes partially during drawing and ironing, resulting innon-workability in the subsequent processing. Accordingly, the sameworking was conducted similarly by using PEI as the non-crystallizablepolyester resin. This PEI, which was non-crystallizable, as expected,exhibited superior characteristics in the drawing step. However, in thesubsequent ironing, the resin adheres to the working punch, (in ironing,usually a metal sheet is pressed into three dies to conduct ironingbetween the punch and the die), causing exfoliation of the resin fromthe inside wall of the can and preventing removal (or strip-out) of thepunch from the can body.

Further, similar drawing-and-ironing working (hereinafter referred to asDI working) was conducted with a simple mixture of PET and PEI, or ofPET, PEI, and PBT with unsatisfactory results. That is, the resincontaining a predominant proportion of a crystalline resin isunsatisfactory in workability, while the resin containing a predominantproportion of a non-crystalline resin was unsatisfactory in strip-outproperty. None of the binary or ternary mixtures of these componentssatisfied simultaneously the workability and strip-out property.

Then it was considered that a simple composition of PET, or PEI, as wellas simple mixtures thereof, show the properties of the respective singlecomponents even though some degree of transesterification may occur onfusion of the resins. Therefore, copolymer resins of terephthalate andisophthalate were prepared, and tested in the same manner, leading tounsatisfactory results. However, in a very limited resin compositionrange, the copolymer resins of terephthalate with isophthalate exhibitedcharacteristics near the target, so that the results were analyzed indetail. Consequently, it was concluded that the control of thecrystallinity only of the resin, as known before, gives neither thedesired adhesiveness to the metal nor the desired workability for severeDI working, but the control of the melting point together with thecrystallinity is indispensable. In other words, it was found that theresin composition which crystallizes only minimally in DI working in anon-crystalline state and has a melting point within a suitabletemperature range can satisfy the object of the present invention.

It was also found that the resin compositions having suchcharacteristics can be prepared by heat fusion of 95-5% by weight ofPET, namely a crystalline polyester, and 5-95% by weight of PEI, namelya non-crystalline polyester in such a state that at least one of thecrystalline polyester and the non-crystallizable polyester iscatalytically active,,and controlling the alloying ratio to come withina predetermined range. The optimum range of the alloy ratio depends onthe nature of the surface to be coated with the resin composition: theratio being from 5 to 50% for a chromate-treated surface, and from 20 to80% for a tin-coated surface.

The state of being "catalytically active" means that the startingpolyester comes to have a higher molecular weight (or a higher limitingviscosity) when heated. The use of such starting material having thecatalytic activity will shorten the time for alloying.

The reasons for the numerical limitations in the first and the secondaspects of the present invention are discussed below.

In alloying the crystalline polyester with the non-crystallinepolyester, the mixing ratio, (non-crystalline polyester/whole polyesterresin)×100!, is limited to be within a range of from 5% to 95%. Themixing ratio is limited to be not less than 5% because, at the ratio ofless than 5%, the crystalline resin exerts great influence on thecrystallization of the resin under the severe ironing condition bystretching, making the resin non-workable. On the other hand, the mixingratio is limited to be not more than 95% because at the ratio of morethan 95%, the apparent melting point is 235° C. or lower to causeadhesion of the resin by fusion to the punch, impairing the strip-outproperty.

Of the thermoplastic polyester resin for coating a chromate-treatedsurface, the alloying ratio is limited to be from 5 to 50% because, atthe alloying ratio of less than 5%, the crystalline portion increases bythe heat generated in the ironing and other steps, making the resinincapable of following the high rate of ironing, and at the alloyingratio of more than 50%, the apparent melting point of the coating resinbecomes lower, impairing greatly the strip-out property.

Of the thermoplastic polyester resin for coating a tin-coated surface,the alloying ratio is limited to be from 20 to 80% because, at thealloying ratio of less than 20%, the crystalline portion increases bythe heat generated in the ironing and other steps, making the resinincapable of following the high rate of ironing, and at the alloyingratio of more than 80%, the apparent melting point of the coating resinbecomes lower, impairing greatly the strip out property.

The reason why the lower limit of the optimum alloying ratio of theresin is lower for a chromate-treated surface than for a tin-coatedsurface is assumed to be that the resins adhere more tightly to thetin-coated surface. The reason why the upper limit of the optimumalloying ratio of the resin is higher for a tin-coated surface than fora chromate-treated surface is assumed to be that the working heatgenerated at ironing on the outside face of the can is absorbed as theheat of fusion of the tin coating layer on the inside face of the can,reducing the temperature rise of the resin of the tin coating layer and,on the contrary, the temperature rise is not reduced on thechromate-treated surface.

Next, the third, the fourth, and the fifth aspects of the invention areexplained below.

As described before, cans were manufactured by coating one side of aknown steel sheet having tin coating on both sides with a variety ofthermoplastic resins by a known method and drawing and ironing the thuscoated steel sheet so that the resin-coated side forms the inside of thecan. Consequently, it was confirmed that the workability and thestrip-out property depend on the coating thermoplastic resin: theworkability being low with PET; the strip-out property being inferiorwith PEI; PET and PEI not giving a steel sheet satisfying both of theworkability and the strip-out property; the mixture of PET and PEI, orPET, PEI and PBI giving inferior workability at a higher content of thecrystalline resin, and giving inferior strip-out property at a highercontent of the non-crystalline resin; and an arbitrary mixture of thesetwo or three components giving no steel sheet satisfying bothworkability and strip-out property.

Accordingly, the idea came to the inventors to laminate thenon-crystalline polyester resin layer having high workability (and highadhesiveness) onto a tin coating layer of the steel sheet and tolaminate further thereon a crystalline polyester resin layer having asatisfactory strip-out property. Therefore, drawing and ironingworkability was investigated by using various non-crystalline polyesterresins and crystalline polyester resins. However, in every combination,layer separation occurred between the lower non-crystallizable polyesterresin layer and the upper non-crystalline polyester resin layer.

Since the phenomenon was assumed to come from the large difference ofthe properties between the lower resin layer and the upper resin layer,the resin compositions of the upper layer and the lower layer wereimproved to prevent the layer separation between the upper and the lowerresin layers and to secure the workability and the strip-out property ofthe resin.

More specifically;

(1) in the case where a non-crystalline polyester resin givingsatisfactory workability (or adhesiveness) is employed for the lowerlayer, a special resin which crystallizes appropriately byresin-stretching at the working is employed for the upper layer resin todecrease the difference of the properties between the upper and thelower resin layers, thereby preventing separation between the upper andthe lower resin layers and securing the strip-out property at theworking;.

(2) in the case where a crystalline resin giving satisfactory strip-outproperty is employed for the upper layer, a special resin whichminimally crystallizes by resin-stretching at the working is employedfor the lower layer resin to decrease the difference of the propertiesbetween the upper and the lower resin layers, thereby preventingseparation between the upper and the lower resin layers and securing theworkability of the resin; and further

(3) the special resin of the above item (1) which crystallizesappropriately by resin-stretching at the working is employed for theupper layer resin, and the special resin of the above item (2) whichminimally crystallizes by resin stretching at the working is employedfor the lower layer resin, thereby decreasing the difference of theproperties between the upper and the lower resin layers, and preventingseparation between the upper and the lower resin layers to secure theworkability of the resin and the strip-out property at the working.

The special resin for the upper layer resin which crystallizesappropriately is a thermoplastic resin of an incomplete reaction productof a crystalline polyester resin with a non-crystalline polyester resin,and has an alloying ratio of 50% or less (excluding 0%) defined by theaforementioned calculation formula.

The special resin for the lower layer resin which crystallizes minimallyis a thermoplastic resin of an incomplete reaction product of acrystalline polyester resin with a non-crystalline polyester resin, andhas an alloying ratio of not less than 50% and less than 100% defined bythe aforementioned calculation formula.

Incidentally, the thermoplastic polyester resin of alloying ratio of100% is a non-crystalline polyester resin, and the thermoplasticpolyester resin of alloying ratio of 0% is a crystalline resin.

The reasons for the numerical limitations in the third, fourth, andfifth aspects of the present invention are discussed below.

The alloying ratio of the thermoplastic polyester resin for the lowerlayer in contact with the tin coating layer of the steel sheet islimited to be not less than 50% and less than 100% because, at thealloying ratio of less than 50%, the adhesiveness of the resin to thetin coating layer is insufficient and the crystalline portion in thecoating resin increases in ironing, especially at an ironing rate of 65%or higher, resulting in loss of adhesiveness to the coating layer and ofworkability of the resin; and at the allowing ratio of 100%, thethermoplastic polyester resin is non-crystalline which cannot preventthe separation of the upper and the lower resin layers.

The alloying ratio of the thermoplastic polyester resin for the upperlayer to be brought into contact with the punch at ironing is limited tobe less than 50% (excluding 0%), because, at the alloying ratio of 50%or higher, a non-crystalline portion predominates in the coating resin,lowering the apparent melting temperature to impair the strip-outproperty, while a thermoplastic polyester resin of alloying ratio of 0%is crystalline which cannot prevent the separation between the upper andthe lower resin layers.

In the thermal fusion reaction of a crystalline polyester resin with anon-crystalline polyester resin for alloying, the mixing ratio, (weightof non-crystalline polyester resin/weight of whole polyesterresin)×100!, is preferably in the range of from 50% to 90% for producinga thermoplastic polyester resin of an alloying ratio of not less than50% and less than 100% for the lower layer; and is preferably in therange of from 10% to 50% for producing a thermoplastic polyester resinof an alloying ratio of less than 50% (excluding 0%), in view of theproduction efficiency.

The coating thicknesses (or lamination thickness) of the upper and thelower resin layers, and the tin coating thickness of the steel sheet arenot limited at all, and the thicknesses may be selected suitablydepending on the working conditions. According to the results of thepractice of the inventors of the present invention, in the cases wherethe tin coating thickness is 2.8 g tin/m² or more and the overallironing ratio is 60% or more, the thickness of the upper layer need benot less than 3 μm before the working. If the thickness is less than 3μm, in that case, a portion of the lower layer resin is fusion-bonded tothe punch by the heat generated at DI working, thus impairing thestrip-out property.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described specifically referring to Examples.

Examples for the First Aspect

A steel sheet (0.30 mm thick, hardness: equivalent to T-1) was employedwhich had been tin-coated in an amount of 2.8 g tin/m² on the side ofthe sheet for an outside face of a can, and had been chromate-treated inan amount of 55 mg metallic chromium/m² and 18 mg chromium oxide/m² onthe side for an inside face of a can. The chromate-treated side of thesteel sheet was coated with the resin composition shown in Table 1 in athickness of 50 μm by means of a T-die. In the coating practice, theresin fusion temperature was in the range of from 265° C. to 300° C.,and the temperature of the steel sheet during the coating was in therange of from 150° C. to 200° C. The steel sheet having been coated withthe resin by the T-die was cooled quickly to 100° C. or lower within 10seconds after the coating. The quick cooling was conducted for thepurpose of preventing crystallization of the resin at a hightemperature.

The resin-coated steel sheet thus produced was drawn and ironed into acan with the resin-coated side forming the inside of the can. Thenondefectiveness of the inside face (the resin coated side) of the canthus manufactured was evaluated by putting aqueous 1.0% sodium chlorideinto the can, applying a voltage of +6 volts employing the can body asan anode and platinum placed at the center position of the can as thecathode, and measuring the electric current. (The test is hereinafterreferred to as a QTV test.) The nondefectiveness of the inside face ofthe can was further evaluated by putting into the can a solutioncontaining 20 g of sulfuric acid per liter and 50 g of copper sulfate(CuSO₄.7H₂ O) per liter, leaving it standing for 10 minutes, removingthe solution from the can, washing the can with water, and observing thedeposition of copper. (The solution is a chemical copper-platingsolution. A defect of the resin layer will induce dissolution of ironfrom the defect if any, causing substitution-plating of copper. The testis hereinafter referred to as a copper sulfate test.)

The results are shown in Table 2 together with the evaluation results ofthe strip-out property and the observation results of the inside face ofthe can after the DI working,

Working Conditions

1. Resin temperature just before DI working:

Normal temperature

2. Blank diameter: 137 mm in diameter

3. Drawing conditions:

First drawing ratio H/D=33/86 mm in diameter

Second drawing ratio H/D=50/65 mm in diameter

4. Diameter of ironing punch:

Three-step ironing: 65.5 mm in diameter

5. Overall ironing ratio: 70.5%

                                      TABLE 1                                     __________________________________________________________________________    Composition of Thermoplastic                                                  Polyester Resin                           Alloying                            Crystalline     Non-Crystalline                                                                       Peak Temperature in DSC (°C.)                                                            Ratio                               Polyester       Polyester                                                                             T.sub.m1                                                                         T.sub.m2                                                                         T.sub.m3                                                                         T.sub.g1                                                                         T.sub.g2                                                                         T.sub.g3                                                                         calcd. (%)                          __________________________________________________________________________    Example 1                                                                           J-125(*1)                                                                          80 wt %                                                                            A(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                                Example 2                                                                           J-125(*1)                                                                          60 wt %                                                                            A(*2)                                                                            40 wt %                                                                            -- -- -- 76.0                                                                             72.2                                                                             75.7                                                                              7.9                                Example 3                                                                           J-125(*1)                                                                          60 wt %                                                                            B(*3)                                                                            40 wt %                                                                            255                                                                              230                                                                              243.7                                                                            -- -- -- 45.2                                Example 4                                                                           J-125(*1)                                                                          60 wt %                                                                            C(*4)                                                                            40 wt %                                                                            -- -- -- 76.0                                                                             90.0                                                                             76.8                                                                              5.7                                Example 5                                                                           J-125(*1)                                                                          40 wt %                                                                            A(*2)                                                                            60 wt %                                                                            -- -- -- 76.0                                                                             70.4                                                                             75.3                                                                             12.5                                Comparative                                                                         J-240(*5)                                                                          100 wt %                                                                           -- --   -- -- -- -- -- -- --                                  Example 1                                                                     Comparative                                                                         J-125(*1)                                                                          78 wt %                                                                            D(*6)                                                                            22 wt %                                                                            -- -- -- 76.0                                                                             72.2                                                                             75.9                                                                              2.6                                Example 2                                                                     Comparative                                                                         --   --   A(*2)                                                                            100 wt %                                                                           -- -- -- -- -- 68.0                                                                             --                                  Example 3                                                                     Comparative                                                                         J-125(*1)                                                                          100 wt %                                                                           -- --   -- -- -- 76.0                                                                             -- -- --                                  Example 4                                                                     Comparative                                                                         J-125(*1)                                                                          90 wt %                                                                            A(*2)                                                                            10 wt %                                                                            255                                                                              238                                                                              245.8                                                                            -- -- -- 54.1                                Example 5                                                                     __________________________________________________________________________     DSC: Differential Thermal Analyzer, Perkin Elmer Model 7                      *1 J125: Crystalline Polyester (inherent viscosity 0.75 dl/g) made by         Mitsui Pet K.K.                                                               *2 A: NonCrystalline Polyester (IA/TA/EG = 50/50/100 mole %, inherent         viscosity 0.85 dl/g)                                                          *3 B: Copolyester (TA/CHDM/EG = 100/30/70 mole %, inherent viscosity 0.80     dl/g) made by Eastman Kodak                                                   *4 C: Polyester Polycarbonate (PC component 25 mole %, inherent viscosity     1.0 dl/g) made by Bayer Co.                                                   *5 J240: LowCrystalline Copolyester (IA/TA/EG = 10/90/100 mole %, inheren     viscosity 0.75 dl/g) made by Mitsui Pet K.K.                                  *6 D: NonCrystalline Copolyester (IA/TA/EG = 90/10/100 mole, % inherent       viscosity 0.85 dl/g)                                                     

                                      TABLE 2                                     __________________________________________________________________________    Strip-Out Property Inside Face  Copper Sulfate Test                           (Adhesion of Resin to Punch                                                                      Observed after                                                                       QTV Value                                                                           (Copper Deposition                                                                     Overall                              and Releasability of Can)                                                                        DI Working                                                                           mA/can                                                                              Observed)                                                                              Evaluation                           __________________________________________________________________________    Example 1                                                                           Good         Good   0.5   None     ⊚                     Example 2                                                                           Good         Good   0.2   None     ⊚                     Example 3                                                                           Good         Good   0.2   None     ⊚                     Example 4                                                                           Good         Good   0.2   None     ⊚                     Example 5                                                                           Good         Good   1.1   None     ⊚                     Comparative                                                                         Good         Bad    50.0  Yes      x                                    Example 1                                                                     Comparative                                                                         Good         Bad    13.0  Yes      x                                    Example 2                                                                     Comparative                                                                         Bad          Good   25.0  Yes      x                                    Example 3                                                                     Comparative                                                                         Good         Bad    65.0  Yes      x                                    Example 4                                                                     Comparative                                                                         Good         Bad    5.0   Yes      x                                    Example 5                                                                     __________________________________________________________________________     ⊚: Excellent                                                   x: Bad                                                                   

Examples for the Second Aspect of the Invention

A steel sheet (0.30 mm thick, hardness: equivalent to T-1) was employedwhich had been tin-coated on both sides, having a tin coating of 2.8g/m² on each side. One side of the tin-coated steel sheet was coatedwith the resin compositions shown in Table 3 in a thickness of 50 μm bymeans of a T-die. In the coating practice, the resin fusion temperaturewas in the range of from 265° C. to 300° C., and the temperature of thesteel sheet during the coating was in the range of from 150° C. to 200°C. The steel sheet coated with the resin by the T-die was cooled quicklyto 100° C. or lower within 10 seconds after the coating. The quickcooling was conducted for the purpose of preventing crystallization ofthe resin at a high temperature and maintaining the resin in anon-crystalline condition.

The resin-coated steel sheet thus produced was drawn and ironed into acan with the resin-coated side forming the inside of the can. Thenondefectiveness of the inside face (the resin coated side) of the canthus manufactured was evaluated by putting aqueous 1.0% sodium chlorideinto the can, applying a voltage of +6 volts employing the can body asan anode and platinum placed at the center position of the can as thecathode, and measuring the electric current. (The test is hereinafterreferred to as a QTV test.) The nondefectiveness of the inside face ofthe can was further evaluated by putting into the can a solutioncontaining 20 g of sulfuric acid per liter and 50 g of copper sulfate(CuSO₄.7H₂ O) per liter, leaving it standing for 10 minutes, removingthe solution from the can, washing the can with water, and observing thedeposition of copper. (The solution is a chemical copper-platingsolution. A defect of the resin layer will induce dissolution of ironfrom the defect if any, causing substitution-plating of copper. The testis hereinafter referred to as a copper sulfate test.)

The results are shown in Table 4 together with the evaluation results ofthe strip-out property and the observation results of the inside face ofthe can after the DI working.

Working Conditions

1. Resin temperature just before DI working:

Normal temperature

2. Blank diameter: 137 mm in diameter

3. Drawing conditions:

First drawing ratio H/D=33/86 mm in diameter

Second drawing ratio H/D=50/65 mm in diameter

4. Diameter of ironing punch:

Three-step ironing: 65.5 mm in diameter

5. Overall ironing ratio: 70.5%

                                      TABLE 3                                     __________________________________________________________________________    Composition of Thermoplastic                                                  Polyester Resin                           Alloying                            Crystalline     Non-Crystalline                                                                       Peak Temperature in DSC (°C.)                                                            Ratio                               Polyester       Polyester                                                                             T.sub.m1                                                                         T.sub.m2                                                                         T.sub.m3                                                                         T.sub.g1                                                                         T.sub.g2                                                                         T.sub.g3                                                                         calcd. (%)                          __________________________________________________________________________    Example 1                                                                           J-125(*1)                                                                          85 wt %                                                                            A(*2)                                                                            15 wt %                                                                            255                                                                              240                                                                              251.7                                                                            -- -- -- 21.5                                Example 2                                                                           J-125(*1)                                                                          80 wt %                                                                            A(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                                Example 3                                                                           J-125(*1)                                                                          60 wt %                                                                            B(*3)                                                                            40 wt %.                                                                           255                                                                              230                                                                              243.7                                                                            -- -- -- 45.2                                Example 4                                                                           J-125(*1)                                                                          40 wt %                                                                            A(*2)                                                                            60 wt %                                                                            -- -- -- 76.0                                                                             70.0                                                                             72.1                                                                             65.0                                Example 5                                                                           J-125(*1)                                                                          20 wt %                                                                            A(*2)                                                                            80 wt %                                                                            -- -- -- 76.0                                                                             69.0                                                                             70.5                                                                             78.0                                Comparative                                                                         J-240(*4)                                                                          100 wt %                                                                           --  0 wt %                                                                            -- -- -- -- -- -- --                                  Example 1                                                                     Comparative                                                                         J-125(*1)                                                                          60 wt %                                                                            A(*2)                                                                            40 wt %                                                                            255                                                                              215                                                                              221                                                                              -- -- -- 85.0                                Example 2                                                                     Comparative                                                                         J-125(*1)                                                                          60 wt %                                                                            A(*2)                                                                            40 wt %                                                                            255                                                                              215                                                                              249                                                                              -- -- -- 15.0                                Example 3                                                                     Comparative                                                                         J-125(*1)                                                                          40 wt %                                                                            A(*2)                                                                            60 wt %                                                                            -- -- -- 76.0                                                                             70.0                                                                             75.4                                                                             10.0                                Example 4                                                                     __________________________________________________________________________     DSC: Differential Thermal Analyzer, Perkin Elmer Model 7                      *1 J125: Crystalline Polyester (inherent viscosity 0.75 dl/g) made by         Mitsui Pet K.K.                                                               *2 A: NonCrystalline Polyester (IA/TA/EG = 50/50/100 mole %, inherent         viscosity 0.85 dl/g)                                                          *3 B: Copolyester (TA/CHDM/EG = 100/30/70 mole %, inherent viscosity 0.80     dl/g) made by Eastman Kodak                                                   *4 J240: LowCrystalline Copolyester (IA/TA/EG = 10/90/100 mole %, inheren     viscosity 0.85 dl/g) made by Mitsui Pet K.K.                             

                                      TABLE 4                                     __________________________________________________________________________    Strip-Out Property Inside Face  Copper Sulfate Test                           (Adhesion of Resin to Punch                                                                      Observed after                                                                       QTV Value                                                                           (Copper Deposition                                                                     Overall                              and Releasability of Can)                                                                        DI Working                                                                           mA/can                                                                              Observed)                                                                              Evaluation                           __________________________________________________________________________    Example 1                                                                           Good         Good   0.9   None     ⊚                     Example 2                                                                           Good         Good   0.5   None     ⊚                     Example 3                                                                           Good         Good   0.4   None     ⊚                     Example 4                                                                           Good         Good   0.2   None     ⊚                     Example 5                                                                           Good         Good   0.2   None     ⊚                     Comparative                                                                         Bad          Bad    50.0  Yes      x                                    Example 1                                                                     Comparative                                                                         Bad          Not Good                                                                             13.0  Yes      x                                    Example 2                                                                     Coniparative                                                                        Good         Bad    25.0  Yes      x                                    Example 3                                                                     Comparative                                                                         Good         Not Good                                                                             5.0   Yes      x                                    Example 4                                                                     __________________________________________________________________________     ⊚: Excellent                                                   x: Bad                                                                   

Examples for the Third to Fifth Aspects of the Invention

A steel sheet (0.30 mm thick, hardness: equivalent to T-1) was employedwhich had been tin-coated on both sides, having a tin coating of 2.8g/m² on each side. One side of the tin-coated steel sheet was coatedwith the thermoplastic polyester resins shown in Table 5 by means of atwo-layer extrusion T-die in a thickness of the lower layer of 20 μm anda thickness of the upper layer of 20 μm (40 μm in total). The resinfusion temperature was in the range of from 265° C. to 300° C., and thetemperature of the steel sheet during the coating was in the range offrom 150° C. to 200° C. The steel sheet having been coated with theresin by the T-die was cooled quickly to 100° C. or lower within 10seconds after the coating. The quick cooling was conducted for thepurpose of preventing crystallization of the resin at a high temperatureand maintaining the resin in a non-crystalline condition.

The resin-coated steel sheet thus produced was drawn and ironed into aDI can with the resin-coated side forming the inside of the can. Thenondefectiveness of the inside face (the resin coated side) of the canthus manufactured was evaluated by putting aqueous 1.0% sodium chlorideinto the can, applying a voltage of +6 volts employing the can body asan anode and platinum placed at the center position of the can as thecathode, and measuring the electric current. (The test is hereinafterreferred to as a QTV test.) The nondefectiveness of the inside face ofthe can was further evaluated by putting into the can a solutioncontaining 20 g of sulfuric acid per liter and 50 g of copper sulfate(CuSO₄.7H₂ O) per liter, leaving it standing for 10 minutes, removingthe solution from the can, washing the can with water, and observing thedeposition of copper. (The solution is a chemical copper-platingsolution. A defect of the resin layer will induce dissolution of ironfrom the defect if any, causing substitution-plating of copper. The testis hereinafter referred to as a copper sulfate test.) The results areshown in Table 6 together with the evaluation results of the strip-outproperty and the observation results of the inside face of the can afterthe DI working.

Working Conditions

1. Resin temperature just before DI working:

Normal temperature

2. Blank diameter: 137 mm in diameter

3. Drawing conditions:

First drawing ratio H/D=33/86 mm in diameter

Second drawing ratio H/D=50/65 mm in diameter

4. Diameter of ironing punch:

Three-step ironing: 65.5 mm in diameter

5. Overall ironing ratio: 70.5%

As described above, the resin-coated steel sheet for DI cans produced byemploying the thermoplastic polyester resin composition of the presentinvention gives DI cans having excellent inside face properties evenwhen drawn and ironed at a normal temperature. Accordingly, thepreheating process in can manufacturing may be omitted, which processhas conventionally been indispensable for DI cans employing conventionalthermoplastic resins, which allows reduction of cost in canmanufacturing.

                                      TABLE 5                                     __________________________________________________________________________              Composition of Thermoplastic                                                  Polyester Resin                     Alloying                        Resin     Crystalline                                                                             Non-Crystalline                                                                       Peak Temperature in DSC (°C.)                                                            Ratio                           Layer     Polyester Polyester                                                                             T.sub.m1                                                                         T.sub.m2                                                                         T.sub.m3                                                                         T.sub.g1                                                                         T.sub.g2                                                                         T.sub.g3                                                                         calcd. (%)                      __________________________________________________________________________    Example 1                                                                           Lower                                                                             J-125(*1)                                                                          30 wt %                                                                            C(*2)                                                                            70 wt %                                                                            -- -- -- 76 70 72.9                                                                             51.0                                  Upper                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                            Example 2                                                                           Lower                                                                             J-125(*1)                                                                          15 wt %                                                                            C(*2)                                                                            85 wt %                                                                            -- -- -- 76 68 70.4                                                                             75.0                                  Upper                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                            Example 3                                                                           Lower                                                                             --   --   C(*2)                                                                            100 wt %                                                                           -- -- -- -- -- -- --                                    Upper                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                            Example 4                                                                           Lower                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            -- -- -- 76 69 71.8                                                                             60.0                                  Upper                                                                             J-125(*1)                                                                          100 wt %                                                                           -- --   -- -- -- -- -- -- --                              Example 5                                                                           Lower                                                                             J-125(*1)                                                                          20 wt %                                                                            C(*2)                                                                            80 wt %                                                                            -- -- -- 76 69 71.8                                                                             60.0                                  Upper                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                            Example 6                                                                           Lower                                                                             J-125(*1)                                                                          20 wt %                                                                            C(*2)                                                                            80 wt %                                                                            -- -- -- 76 69 71.8                                                                             60.0                                  Upper                                                                             J-125(*1)                                                                          60 wt %                                                                            D(*3)                                                                            40 wt %                                                                            255                                                                              235                                                                              243.7                                                                            -- -- -- 45.2                            Comparative                                                                         Lower                                                                             J-240(*4)                                                                          100 wt %                                                                           -- --   -- -- -- -- -- -- --                              Example 1                                                                           Upper                                                                             J-240(*4)                                                                          100 wt %                                                                           -- --   -- -- -- -- -- -- --                              Comparative                                                                         Lower                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                            Example 2                                                                           Upper                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                            Comparative                                                                         Lower                                                                             J-125(*1)                                                                          60 wt %                                                                            D(*3)                                                                            40 wt %                                                                            255                                                                              230                                                                              243.7                                                                            -- -- -- 45.2                            Example 3                                                                           Upper                                                                             J-125(*1)                                                                          80 wt %                                                                            C(*2)                                                                            20 wt %                                                                            255                                                                              235                                                                              249.7                                                                            -- -- -- 26.5                            Comparative                                                                         Lower                                                                             J-125(*1)                                                                          40 wt %                                                                            C(*2)                                                                            60 wt %                                                                            -- -- -- 76 71 73.0                                                                             60.1                            Example 4                                                                           Upper                                                                             J-125(*1)                                                                          90 wt %                                                                            C(*2)                                                                            10 wt %                                                                            255                                                                              238                                                                              245.8                                                                            -- -- -- 54.1                            __________________________________________________________________________     DSC: Differential Thermal Analyzer, Perkin Elmer Model 7                      *1 J125: Crystalline Polyester (inherent viscosity 0.75 dl/g) made by         Mitsui Pet K.K.                                                               *2 C: NonCrystallizable Polyester (IA/TA/EG = 50/50/100 mole %,               glasstransition temperature, 68° C., inherent viscosity 0.85 dl/g)     made by Mitsui Pet K.K.                                                       *3 D: Copolyester (TA/CHDM/EG = 100/30/70 mole %, inherent viscosity 0.80     dl/g) made by Eastman Kodak                                                   *4 J240: LowCrystalline Copolyester (IA/TA/EG = 10/90/100 mole %, inheren     viscosity 0.85 dl/g) made by Mitsui Pet K.K.                             

                                      TABLE 6                                     __________________________________________________________________________    Strip-Out Property Inside Face  Copper Sulfate Test                           (Adhesion of Resin to Punch                                                                      Observed after                                                                       QTV Value                                                                           (Copper Deposition                                                                     Overall                              and Releasability of Can)                                                                        DI Working                                                                           mA/can                                                                              Observed)                                                                              Evaluation                           __________________________________________________________________________    Example 1                                                                           Good         Good   0.7   None     ⊚                     Example 2                                                                           Good         Good   0.5   None     ⊚                     Example 3                                                                           Good         Good   0.5   None     ⊚                     Example 4                                                                           Good         Good   0.2   None     ⊚                     Example 5                                                                           Good         Good   1.0   None     ⊚                     Example 6                                                                           Good         Good   1.1   None     ⊚                     Comparative                                                                         Bad          Bad    50.0  Yes      x                                    Example 1                                                                     Comparative                                                                         Good         Bad    23.0  Yes      x                                    Example 2                                                                     Comparative                                                                         Good         Not Good                                                                             18.0  Yes      x                                    Example 3                                                                     Comparative                                                                         Not Good     Bad    34.0  Yes      x                                    Example 4                                                                     __________________________________________________________________________     ⊚: Excellent                                                   x: Bad                                                                   

What is claimed is:
 1. A resin-coated steel sheet for DI cans, obtainedby a method comprising:providing a mixture containing 95 to 5% by weightof a crystalline polyester resin and 5 to 95% by weight of anon-crystalline polyester resin; reacting said resins, by means ofthermal fusion, to produce a first thermoplastic polyester resin havingan alloying ratio, as represented by one of formulas (a) or (b) below,of not less than 50% and less than 100% and a second thermoplasticpolyester resin having an alloying ratio, as represented by one offormulas (a) or (b) below, of greater than 0% and less than 50% where(a) and (b) are as follows: ##EQU2## where T_(m1) and T_(g1) arerespectively the melting point and the glass-transition temperature, in° C., of the crystalline polyester resin, T_(m2) and T_(g2) arerespectively the melting point and the glass-transition temperature, in° C., of a random copolymer of a polyester resin having a monomercomposition equal to that of the respective first and secondthermoplastic polyester resins, T_(m3) and T_(g3) are respectively themelting point and the glass-transition temperature in ° C., of therespective first and second thermoplastic polyester resins; coating thefirst and second thermoplastic polyester resins by means of a two-layerextrusion T-die onto one side of a two-sided steel sheet tin-coated onboth sides, so that a layer of the first thermoplastic polyester resinis interposed between the tin-coated steel sheet and a layer of thesecond thermoplastic polyester resin, wherein said first thermoplasticpolyester resin retains a degree of crystallinity low enough to assuresatisfactory adhesion to the tin-coated steel sheet, even after DIworking, while said second thermoplastic polyester resin is innon-crystalline condition and is to be converted by DI working into acrystalline resin layer to assure a satisfactory stripping-out property;and cooling the coated resins quickly to prevent further crystallizationof the layers of the first and second thermoplastic polyester resins. 2.A process for producing a resin-coated steel sheet for DI cans,comprising:providing a mixture containing 95 to 5% by weight of acrystalline polyester resin and 5 to 95% by weight of a non-crystallinepolyester resin; reacting said resins, by means of thermal fusion, toproduce a first thermoplastic polyester resin having an alloying ratio,as represented by one of formulas (a) or (b) below, of not less than 50%and less than 100% and a second thermoplastic polyester resin having analloying ratio as represented by one of formulas (a) or (b) below, ofgreater than 0% and less than 50% where (a) and (b) are as follows:##EQU3## where T_(m1) and T_(g1) are respectively the melting point andthe glass-transition temperature, in ° C., of the crystalline polyesterresin, T_(m2) and T_(g2) are respectively the melting point and theglass-transition temperature, in ° C., of a random copolymer of apolyester resin having a monomer composition equal to that of therespective first and second thermoplastic polyester resins, T_(m3) andT_(g3) are respectively the melting point and the glass-transitiontemperature, in ° C., of the respective first and second thermoplasticpolyester resins; coating the first and second thermoplastic polyesterresins by means of a two-layer extrusion T-die onto one side of atwo-sided steel sheet tin-coated on both sides, so that a layer of thefirst thermoplastic polyester resin is interposed between the tin-coatedsteel sheet and a layer of the second thermoplastic polyester resin,wherein said first thermoplastic polyester resin retains a degree ofcrystallinity low enough to assure satisfactory adhesion to thetin-coated steel sheet, even after DI working, while said secondthermoplastic polyester resin is in non-crystalline condition and is tobe converted by DI working into a crystalline resin layer to assure asatisfactory stripping-out property; and cooling the coated resinsquickly to prevent further crystallization of the layers of the firstand second thermoplastic polyester resins.
 3. A drawn and ironed canproduced by drawing and ironing a resin-coated steel sheet, wherein theresin-coated steel sheet is obtained by a method comprising:providing amixture containing 95 to 5% by weight of a crystalline polyester resinand 5 to 95% by weight of a non-crystalline polyester resin; reactingsaid resins, by means of thermal fusion, to produce a firstthermoplastic polyester resin having an alloying ratio, as representedby one of formulas (a) or (b) below, of not less than 50% and less than100% and a second thermoplastic polyester resin having an alloyingratio, as represented by one of formulas (a) or (b) below, of greaterthan 0% and less than 50% where (a) and (b) are as follows: ##EQU4##where T_(m1) and T_(g1) are respectively the melting point and theglass-transition temperature, in ° C., of the crystalline polyesterresin, T_(m2) and T_(g2) are respectively the melting point and theglass-transition temperature, in ° C., of a random copolymer of apolyester resin having a monomer composition equal to that of therespective first and second thermoplastic polyester resins, T_(m3) andT_(g3) are respectively the melting point and the glass-transitiontemperature, in ° C., of the respective first and second thermoplasticpolyester resins; coating the first and second thermoplastic polyesterresins by means of a two-layer extrusion T-die onto one side of atwo-sided steel sheet tin-coated on both sides where the one side withthe first and second resins forms the inside of the can, so that a layerof the first thermoplastic polyester resin is interposed between thetin-coated steel sheet and a layer of the second thermoplastic polyesterresin, wherein said first thermoplastic polyester resin retains a degreeof crystallinity low enough to assure satisfactory adhesion to thetin-coated steel sheet, even after DI working, while said secondthermoplastic polyester resin is in non-crystalline condition and is tobe converted by DI working into a crystalline resin layer to assure asatisfactory stripping-out property; and cooling the coated resinsquickly to prevent further crystallization of the layers of the firstand second thermoplastic polyester resins.
 4. A process for producing adrawn and ironed can, comprising:providing a mixture containing 95 to 5%by weight of a crystalline polyester resin and 5 to 95% by weight of anon-crystalline polyester resin; reacting said resins, by means ofthermal fusion, to produce a first thermoplastic polyester resin havingan alloying ratio, as represented by one of formulas (a) or (b) below,of not less than 50% and less than 100% and a second thermoplasticpolyester resin having an alloying ratio, as represented by one offormulas (a) or (b) below, of greater than 0% and less than 50% where(a) and (b) are as follows: ##EQU5## where T_(m1) and T_(g1) arerespectively the melting point and the glass-transition temperature, in° C., of the crystalline polyester resin, T_(m2) and T_(g2) arerespectively a melting point and a glass-transition temperature, in ° C.of a random copolymer of a polyester resin having a monomer compositionequal to that of the respective first and second thermoplastic polyesterresins, T_(m3) and T_(g3) are respectively the melting point and theglass-transition temperature, in ° C., of the respective first andsecond thermoplastic polyester resins; coating the first and secondthermoplastic polyester resins by means of a two-layer extrusion T-dieonto the side of a two-sided steel sheet tin-coated on both sides, sothat a layer of the first thermoplastic polyester resin is interposedbetween the tin-coated steel sheet and a layer of the secondthermoplastic polyester resin, wherein said first thermoplasticpolyester resin retains a degree of crystallinity low enough to assuresatisfactory adhesion to the tin-coated steel sheet, even after DIworking, while said second thermoplastic polyester resin is innon-crystalline condition and is to be converted by DI working into acrystalline resin layer to assure a satisfactory stripping-out property;cooling the coated resins quickly to produce a resin-coated steel sheetand to prevent further crystallization of the layers of the first andsecond thermoplastic polyester resins; and drawing and ironing theresultant resin-coated steel sheet to form a can, with the resin-coatedside of the steel sheet forming the inside of the can.